The 3rd-Generation Partnership Project (3GPP) is currently developing a next-generation wireless communication system in its Long-Term Evolution (LTE) initiative. A key requirement for LTE is frequency flexibility. To this end, multiple carrier bandwidths ranging from 1.4 MHz to 20 MHz are supported. In addition, frequency-division duplexing (FDD) and time-division duplexing (TDD) are both supported, so that either paired or unpaired spectrum may be used in a particular area. In FDD systems, frequencies used in the downlink (the link from the base stations to the mobile terminals) and the uplink (the link from mobile terminals to base stations) differ. Thus, uplink and downlink transmissions between a base station and a mobile station may be simultaneous. In TDD systems, however, uplink and downlink transmissions use the same frequency band, and may not occur simultaneously. Hence, the available resources must be divided in time between the uplink and the downlink. By allocating different proportions of time (generally in terms of the number of subframes of a radio frame) to uplink and downlink, it is possible to adapt to asymmetric traffic and resource needs in the uplink and downlink.
This potential asymmetry between uplink and downlink resources leads to another significant difference between FDD and TDD. While the same number of uplink and downlink subframes is available during a radio frame in an FDD system, the number of uplink and downlink subframes may differ for a TDD system. In an FDD system, every downlink subframe can be associated with an uplink subframe, which allows feedback to be sent in a one-to-one fashion. Thus, it is possible for a mobile terminal to send feedback in response to each downlink assignment of resources in a corresponding uplink subframe, subject only to a processing delay. In a TDD system, however, the uplink and downlink are discontinuous, and, more importantly, may be unbalanced in a given radio frame. Thus, it may not be possible to construct a one-to-one association, even with the help of a certain delay. For the typical case with more downlink subframes than uplink subframes, it is often the case that the feedback from two or more downlink subframes must be transmitted in a single uplink subframe. Error control techniques that account for these differences must therefore be implemented.